Before leaving this section we should mention that a purist might say that the real prediction of single \(j\)  is that all the charge quadrupole moments are zero because the neutrons have no charge. We have to assign an effective charge to the neutrons, popular choice being \(e_{\text{eff}}=0.5\) . But note that even the large space calculations including those of [1]} require effective charges in order to get agreement with experiment. In first order perturbation theory the effective charge comes from \(\Delta N =2\)  excitations. For example for \(^{41}\) Ca excitations from 0p to 1p; from 0d to 0g, 1d, and 2s. As large as model spaces are in [1]} and in nearly all other calculations these configurations are not present and one needs to insert effective charges.
